Key Points

The human eye works like a camera, where the lens system forms a real, inverted image on a light-sensitive screen called the retina. Light enters through the cornea and the amount is regulated by the pupil.

The ability of the eye lens to adjust its focal length, with the help of ciliary muscles, to focus on both distant and nearby objects is called accommodation.

The near point, or least distance of distinct vision, is about 25 cm for a young adult. The far point, the farthest point an eye can see clearly, is at infinity for a normal eye.

A person with myopia can see nearby objects clearly but not distant ones because the image is formed in front of the retina. This defect is corrected using a concave lens of suitable power.

A person with hypermetropia can see distant objects clearly but not nearby ones because the image is formed behind the retina. This defect is corrected using a convex lens of appropriate power.

This is an age-related defect where the power of accommodation decreases due to weakening ciliary muscles and reduced lens flexibility. It is often corrected using bifocal lenses (concave and convex).

When a ray of light passes through a glass prism, it bends at both surfaces. The emergent ray deviates at an angle to the incident ray, which is called the angle of deviation ($\angle D$).

Dispersion is the splitting of white light into its band of seven colors (VIBGYOR) when it passes through a prism. Violet light bends the most, and red light bends the least.

Isaac Newton showed that an inverted prism can recombine the spectrum of colors produced by a first prism back into a beam of white light. This proved sunlight is made of seven colors.

A rainbow is a natural spectrum caused by the dispersion, internal reflection, and final refraction of sunlight by water droplets in the atmosphere. It always forms in the direction opposite to the Sun.

The refraction of light by the Earth's atmosphere, due to its varying optical densities, causes phenomena like the twinkling of stars and advanced sunrise.

The continuous refraction of starlight by a constantly changing atmosphere causes the apparent position of the star to fluctuate, making it twinkle. Planets do not twinkle as they are extended sources of light.

Due to atmospheric refraction, the Sun is visible about 2 minutes before the actual sunrise and about 2 minutes after the actual sunset.

The scattering of light by colloidal particles makes the path of a light beam visible, which is known as the Tyndall effect. The color of scattered light depends on the size of the particles.

Fine particles in the atmosphere are more effective at scattering shorter wavelengths (blue end of the spectrum) than longer ones. The scattered blue light enters our eyes, making the sky appear blue.

At sunrise and sunset, light travels a longer distance through the atmosphere, scattering away most of the blue light. The longer wavelength light, like red and orange, reaches our eyes, making the Sun appear reddish.